1. Technical Field
The present invention is directed to footwear, and in particular, to a shoe sole having a sole including a sealed, fluid-filled viscoelastic cushioning element such as a gas-filled bladder.
2. The Prior Art
Footwear including soles made of a resiliently compressible midsole disposed above a substantially flexible, wear-resistant outsole are known in the art. Such midsoles have been made of conventional foam materials, for example, ethylene vinyl acetate (EVA) or polyurethane which compress resiliently under an applied load and dampen to provide cushioning. The outsoles have been made of conventional wear-resistant materials such as a carbon-black rubber compound. Conventional foam materials are resiliently compressible, in part, due to the inclusion in the foam of open or closed cells defining an inner volume that is substantially displaced by gas. That is, the foam can include bubbles formed in the material which include air therein. However, after repeated compression, foam materials deteriorate, in particular, by compaction. The cell structure collapses, resulting in decreased compressibility of the foam. Thus, the overall cushioning of the midsole deteriorates.
One way to overcome the drawbacks of using conventional foam materials is disclosed in U.S. Pat. No. 4,183,156, incorporated by reference, in which cushioning is provided by inflatable inserts made of elastomeric materials. The inserts include a plurality of tubular chambers which extend substantially longitudinally throughout the length of the shoe. The chambers are in fluid communication with each other at the forefoot and jointly extend across the width of the shoe. In one embodiment,-the insert is disposed upon a relatively thick outsole, within the shoe upper. A moderator is placed over the insert and also is disposed within the shoe upper. The moderator is made of a semi-flexible material which allows it to conform to the changing contours of the plantar surface of the wearer's foot. The shoe upper is secured to upper surfaces of the outsole such that the outsole, the lower side portions of the shoe upper and the moderator define a chamber in which the insert is contained. In a second embodiment, the insert is disposed within a cavity formed in an elastic portion of the outsole.
In each of the above-described embodiments, cushioning is provided, at least in part, by resilient compression of the elastic insert. However, in the first embodiment, compression of the insert requires relative movement of the foot with respect to the upper, within the volume defined by the upper, moderator and outsole. Relative movement between the foot and the upper can cause callouses, blisters and other problems. Further, positioning of the insert within the upper inherently is restrictive, for example, the degree to which the insert may elastically deform under load is inhibited. Thus, the degree to which the insert can be compressed and the degree of cushioning which can be provided thereby is limited. In the second embodiment, the overall cushioning achieved by the sole is due in part to the foam elastic material which is disposed about the inflated insert. Thus, when the foam material deteriorates, the quality of cushioning afforded by the shoe can decline. As with the upper in the first embodiment, the elastic material disposed about the insert can restrict the degree to which it can compress, thereby limiting cushioning.
In U.S. Pat. No. 4,219,945, incorporated by reference, an inflated insert may be encapsulated in a foam material. The combination of the insert and the encapsulating material functions as the midsole. An upper is cemented to the upper surface of the encapsulating material and an outsole or tread member may be fixed to the lower surface. As with the second embodiment of the above-discussed patent, the degree to which the insert may compress and thus the cushioning provided thereby, is limited by the encapsulating material. Overall cushioning can decrease as the foam material deteriorates with use, and the use of a completely encompassing foam increases the weight of the shoe. Further, the impact response of such a sole structure is determined by the combined effects of both the insert and the foam material. Factors such as the relative volume of the two elements, the type of foam material used and the pressure of the enclosed gas varies the amount each element contributes to the impact response and the nature of the response. Accordingly, the use of both foam material and an insert can complicate engineering the cushioning response to particular requirements.
U.S. Pat. No. 4,817,304, incorporated by reference, discloses a foam encapsulated air insert in which gaps are left along the sides of the encapsulating member. When the midsole is compressed, the insert expands into the gaps. Thus, the gaps provide decreased stiffness in compression of the midsole, while reducing the overall weight of the shoe. Further, by appropriately locating the gaps, the overall impact response characteristics can be tuned along the length of the shoe. However, as with the above-discussed patent, the use of a substantial quantity of foam material throughout the shoe midsole increases the weight of the shoe, inhibits flexibility and increases the stiffness in compression of the midsole at locations other than the gaps. Further, the midsole suffers the drawback of deterioration of the overall cushioning as the foam material degrades with use.
U.S. Pat. No. 4,722,131, incorporated by reference, discloses an air cushioning sole made of an elastomeric material which is disposed between an upper and a shoe bottom sole. The cushioning sole may be formed by blow molding and in one embodiment, includes two separate sections which jointly extend across the length and width of the shoe. Each section includes a separate air valve to allow the cavities to be inflated to a different pressure.
In one embodiment, the cushioning sole includes a forefoot section formed to include a forefoot chamber extending along the medial and lateral sides of the forefoot, and a plurality of transverse chambers extending across the shoe between the lateral and medial portions of the forefoot chamber. The sole also includes a separate heel section, and a separate air valve to inflate each section. The transverse chambers are in fluid communication with each other by small connecting tubes formed between each transverse tube along the longitudinal axis of the cushioning sole, with the forward and rear transverse chamber linked to the forefoot chamber.
Since the forefoot chamber is in fluid communication with the transverse chambers, the cushioning area covered by the forefoot cavity is essentially the entire forefoot of the shoe. Thus the effective volume of the cushioning sole at the forefoot is large relative to the area of the shoe it covers, potentially making the cushioning sole unstable. This instability may be overcome by greatly increasing the pressure within the cushioning sole. However, increasing the pressure to a level which overcomes the instability can result in a cushioning sole having a greatly increased stiffness in compression, thereby making the cushioning sole too firm to provide an acceptable level of cushioning. Further, large increases in pressure will cause the walls of the chambers to distend, forming an uneven surface which can require that the cushioning sole be foam encapsulated, which results in the further drawbacks discussed above.